1. Field of the Invention
This invention relates to an in-cylinder injection type spark-ignition internal combustion engine in which fuel is injected directly into a combustion chamber and ignited by a spark plug.
2. Description of the Related Art
In the in-cylinder injection type spark-ignition internal combustion engine in which fuel is injected directly into a combustion chamber, stratified lean combustion can be carried out, where ignition is made at an overall very lean air/fuel ratio, for example, by transferring fuel spray injected by a fuel injection valve in the compression stoke to near an electrode part of a spark plug, and thereby forming a mixture having an air/fuel ratio close to the stoichiometric air/fuel ratio around the electrode part of the spark plug. There are various manners of transferring the fuel spray to near the electrode part of the spark plug. An internal combustion engine arranged such that a change between different manners of transferring the fuel spray (hereinafter referred to as “spray transfer modes”) is carried out depending on the operational conditions of the engine has been proposed by Japanese Unexamined Patent Publication No. Hei 11-210472 (hereinafter referred to as “patent document 1”), for example.
In the technique disclosed in patent document 1, a fuel injection valve is arranged in an almost vertical position at the top of a combustion chamber, a spark plug is arranged with its electrode part located near and facing the injection hole part of the fuel injection valve, and a cavity is formed in the top face of a piston. When the engine is in a low engine-load operational region, stratified combustion is carried out by a so-called spray-guide method, in which, by retarding the fuel injection timing to be close to the ignition timing, it is arranged that ignition is made at the time when the fuel spray from the fuel injection valve arrives near the electrode part of the spark plug on its own kinetic energy. Meanwhile, when the engine is in a high engine-load operational region, stratified combustion is carried out by a so-called wall-guide method, in which, by advancing the fuel injection timing, it is arranged that the fuel spray is transferred to near the electrode part of the spark plug with the help of a tumbling flow produced by the cavity of the piston and ignited.
In order to change the spray transfer mode depending on the load on the internal combustion engine as in the technique disclosed in patent document 1, it is necessary to change the fuel injection timing and ignition timing using a specified engine torque as a threshold. However, in the control using normal fuel injection timing maps and normal ignition timing maps, in order to prevent a step in torque due to an abrupt change in fuel injection timing and ignition timing, the fuel injection timing and ignition timing are changed continuously, by performing interpolation to the control maps. This causes a problem that at the time of changing a spray transfer mode, misfire occurs due to the following reasons:
FIG. 4 is a characteristic diagram which relates to a specified air/fuel ratio and shows, in respect of fuel injection timing Tij (horizontal axis) and ignition timing Tig (vertical axis), a region capable of achieving stable stratified combustion by the spray-guide method (SG region) and a region capable of achieving stable stratified combustion by the wall-guide method (WG region). FIG. 4 shows that at this air/fuel ratio, the SG region and the WG region are independent of each other, with a misfire region between them. Hence, if the fuel injection timing Tij and ignition time Tig are changed continuously by interpolation as mentioned above, there can be cases where the fuel injection timing and ignition time stay long in the misfire region so that misfire continues. In order to avoid this situation, a spray transfer mode needs to be changed depending on the engine load, however, the change of the mode cannot be carried out. Thus, the merits of both fuel transfer modes, such as improvement in fuel economy and reduction in NOx for example, cannot be fully utilized.